A Novel Methodology to Synthesize Highly Conductive Anion Exchange
Membranes
Yubin He,† Jiefeng Pan,† Liang Wu, Yuan Zhu, Xiaolin Ge, Jin Ran, ZhengJin Yang, and Tongwen
Xu*
CAS Key Laboratory of Soft Matter Chemistry, Collaborative Innovation Center of Chemistry for Energy
Materials, School of Chemistry and Material Science, University of Science and Technology of China,
Hefei 230026, P.R. Tel.: +86-551-6360-1587.E-mail:twxu@ustc.edu.cn.
† These authors contribute equally.
*
Corresponding author: Tel.: +86-551-6360-1587.E-mail:twxu@ustc.edu.cn.
Materials:
Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) with an intrinsic viscosity of 0.57 dl/g in
chloroform at 25 oC was obtained from Tianwei Membrane Company (Shandong, P.R. China). Nbromosuccinimide (NBS), 2,2'-azobis(2-methylpropionitrile) (AIBN), N,N,N',N'-Tetramethyl-1,6hexanediamine (TMHDA), bromoethane and 1,4-dibromobutane was purchased from Energy
Chemical Co. Ltd.(Shanghai, P.R. China) and used as received. N-methyl-2-pyrrolidolone (NMP,
AR), tetrahydrofuran (THF), CH3CN, acetone, ether, chlorobenzene, ethanol, chloroform,
trimethylamine aqueous solution (33 %), Hydrochloric acid (HCl) aqueous solution (37% AR),
sodium chloride (AR), sodium hydroxide (AR) and sodium sulfate (Na2SO4, AR) were purchased
from Sinopham Chemical Reagent Co. Ltd. Deionized water was used throughout.
Bromination of poly (2,6-dimethyl-1,4-phenylene oxide) (BPPO):
PPO was brominated via free radical bromination as previously reported1. A typical synthetic
procedure is described as follows: To a stirred solution of PPO (6 g, 50 mmol) in chlorobenzene (60
mL) was added N-bromosuccinimide (NBS) (3.6 g, 20 mmol) and 2,2-azobis(2-methylpropionitrile)
(0.3 g). The reaction mixture was heated at 140 oC for 8 hours. Afterwards, it was poured into excess
ethanol to form light brown precipitate of BPPO. Then, the polymer was redissolved in chloroform
(60 mL) followed by precipitation in ethanol again. After dried at 60 oC for 48 hours, BPPO was
obtained as light brown fibers with a bromination degree of 0.23 (Characterized by NMR employing
CDCl3 as solvent).
Synthesis of 6-(dimethylamino)-N-ethyl-N,N-dimethylhexan-1-aminium bromide (DMAQA)2:
1
DMAQA was synthesized as previously reported with minor modifications. As depicted in Figure
S1, 2.1 mL bromoethane in 15 mL ethanol was added to a stirred solution of 6 mL TMHDA in 75 mL
ethanol under nitrogen atmosphere. The reaction was carried out at room temperature for 48 hours.
Ethanol was evaporated in vacuum and the residue was washed by ether for several times. The white
precipitate was collected by filtration and then added to 50 mL of acetone. After filtration, acetone
was removed in vacuum and the product was obtained as white hygroscopic powder. Pure DMAQA
was obtain by recrystallization from acetone/ether mixture (Yield=37 %). NMR spectra of DMAQA
were obtained employing D2O as solvent.
Synthesis of 4-bromo-N,N,N-trimethylbutan-1-aminium bromide (BrQA)3:
BrQA was synthesized as previously reported as depicted in Figure S1. To a stirred solution of 1,4dibromobutane (10 mL) in THF (100 mL) was continuously bumbled dry trimethylamine gas at
ambient temperature and pressure conditions for 3 hours. Afterwards, the reaction mixture was stirred
at room temperature for 48 hours. The white precipitate was collected by filtration and washed with
ether followed by dried in vacuum for 48 hours (Yield=89 %). NMR spectra of BrQA were obtained
employing D2O as solvent.
Figure S1. Synthetic procedures of DMAQA and DMABQA.
2
Figure S2. 1H NMR spectra of DMAQA.
Synthesis
of
N1-(6-(dimethylamino)hexyl)-N1,N1,N4,N4,N4-pentamethylbutane-1,4-diaminium
bromide (DMABQA)4:
A solution of 2 g BrQA and 10 mL TMHDA in 30 mL acetonitrile was heated at 60 oC for 24 hours.
After filtration, the filtrate was concentrated in vacuum followed addition of excess ether. Afterward,
the precipitate was collected by filtration and washed by ether to yield DMABQA as a white powder.
It was purified by recrystallization and then dried at 60 oC in vacuum (Yield=45 %). NMR spectra of
DMABQA were obtained employing D2O as solvent.
Figure S3. 1H NMR spectra of BrQA and DMABQA.
Synthesis of multi-cations functionalized AEMs (BQAPPO and TQAPPO):
3
To a stirred solution of 1 g BPPO in 10 mL NMP was added 1.2 equiv of DMAQA or DMABQA
followed by stirring at room temperature for 24 hours. Afterwards, it was poured into excess ether and
synthesized polyelectrolyte was collected by filtration and washed with ether for several times. After
dried at 60 oC for 24 hours, the polymer (1 g) was dissolve in NMP (15 mL) then casted onto glass
plate and heated at 60 oC to form transparent membrane. Key properties of the synthesized AEMs
were listed in Table S1. NMR spectrums of BQAPPO and TQAPPO were obtained employing
CD3OD as solvent.
Table S1. Key properties of synthesized AEMs.
Grafting
IEC a)
Water uptake b)
Swelling c)
Conductivity d)
ratio
(mmol/g)
(%)
(%)
(mS/cm)
QPPO-0.25
0.25
1.47
48.7
9.1
12.9
QPPO-0.27
0.27
1.62
73.8
15.5
18.6
QPPO-0.33
0.33
1.88
99.5
21.1
29.5
QPPO-0.36
0.36
1.99
143.6
30.3
32.9
BQAPPO-0.10
0.10
1.22
8.9
3.2
16.5
BQAPPO-0.12
0.12
1.37
18.2
3.8
21.6
BQAPPO-0.15
0.15
1.56
42.5
11.1
36.3
BQAPPO-0.23
0.23
2.13
120.4
24.1
53
TQAPPO-0.10
0.10
1.72
55.6
9.2
43.3
TQAPPO-0.12
0.12
1.93
67.2
15.7
50.3
TQAPPO-0.15
0.15
2.07
76.2
16.6
57.9
TQAPPO-0.17
0.17
2.24
86.2
18.9
69.2
Membrane
a) Measured by Mohr method. b) Measured at 25 oC, OH- forms. c) In plane swelling ratio at 25 oC in
OH- form. d) Measure at 25 oC in fully hydrated conditions.
Characterizations:
1
H NMR spectra was performed with an AV III 400 NMR spectrometer ( 1H resonance at 400
MHz, Bruker). Tapping mode atomic force microscopy (AFM) was recorded by a Veeco
diinnova SPM, using micro-fabricated cantilevers with a force constant of approximately 20 N m1
. Mechanical properties of synthesized AEMs were measured by a DMA Q800 V20.24 Build 43.
Water uptake, swelling ratio and ion exchange capacity
4
A sample of synthesized AEM (1 cm in width and 4 cm in length) with a given mass was
immersed in deionized water at room temperature for 24 hours. After that, the sample was taken
out and excess water on the surface was removed by wiping the membrane with tissue paper.
Length and mass of the hydrated membrane were measured quickly and liner expansion ratio
(LER) was calculated as follows:
LER 
Lw  Ld
 100%
Ld
(1)
Lw and Ld were defined as the length of the sample in hydrated and dehydrated conditions
separately. Similarly, water uptake (WU) of the membrane can be calculated by the following
equation:
WU 
Ww  W d
 100%
Wd
(2)
where Ww and Wd were defined as the mass of the sample in hydrated and dehydrated
conditions separately.
Ion exchange capacity (IEC) of the prepared AEMs was measured by the conventional
Mohr method. One sample of the membranes was firstly immersed in 1 mol/L NaCl aqueous
solution at room temperature, then dried at 80 oC in vacuum overnight. After the mass of the
sample was recorded, it was immersed in 0.5 mol/L Na2SO4 aqueous solution for another 24
hours to release Cl- from the membrane. Lastly, the solution was titrated with 0.1 mol/L AgNO3
aqueous solution employing K2CrO4 as indicator. IEC was calculated as follows:
IEC (mmol / g ) 
V Ag (mL )  0.1(mol / L )

W Cl (g )
(3)

was the amount of AgNO3 solution consumed while titration and
was the mass of
the sample in Cl- form.
Hydroxide Conductivity
Hydroxide conductivity of synthesized AEMs was measured by conventional four-point probe
technique employing an Autolab PGSTAT 30 (Eco Chemie, Netherland) in galvanostatic mode
and with an a.c. current amplitude of 0.1 mA and a frequency range of 1 MHz to 100 Hz. Bode
plots were used to determine the frequency region over which the magnitude of the impedance
5
was constant. Afterwards, the ionic resistance was obtained from the associated Nyquist plot. A
sample of synthesized AEM was set into a Teflon cell and the membrane was in contact with 2
current collecting electrodes and 2 potential sensing electrodes (the distance between the potential
sensing electrodes was 1 cm). Then, the cell was quickly immersed in deionized water in order to
minimize the potential error caused by reaction of the hydroxide ions in the AEM with dissolved
carbon dioxide. Afterwards, the impedance spectrum was collected. The ionic conductivity (κ)
was calculated according to the following equation:
 
L
RWD
(4)
where L is the distance between potential sensing electrodes, R is the membrane resistance, d
and W are the thickness and width (1 cm) of AEM sample respectively. The samples were
equilibrated at the pre-determined temperatures for at least 30 min before measurement.
Estimation of the alkaline stabilities
A sample of synthesized AEM was soaked in aqueous 1 mol/L KOH solution at 60 oC for
increasing lengths of time. Afterwards, the membranes were immersed in distilled water and
wash frequently for 48 h to remove the residual KOH. Afterwards, hydroxide conductivities and
mechanical properties of the samples were again measured.
Table S2. Mechanical properties of QPPO, BQAPPO and TQAPPO AEMs (in OH- form).
Membrane
IEC (mmol/g)
TS (MPa) 1)
Eb (%) 1)
TS (MPa)
Eb (%)
QPPO-0.36
1.99
8.36
9.43
brittle 3)
brittle 3)
BQAPPO-0.23
2.13
5.55
19.5
3.77 2)
20.2 2)
TQAPPO-0.17
2.24
6.55
55.1
3.42 2)
25.0 2)
1) Measured before alkaline treatment. 2) Measured after soaking in 1 M KOH for 15 days.
3) Measured after soaking in 1 M KOH for 10 days.
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